Add RIL_UNSOL_KEEPALIVE_STATUS in ril_unsol_commands.h

Bug: 134048171
Test: 1. Build Pass
      2. Running atest android.net.cts.ConnectivityManagerTest
         #testSocketKeepaliveLimitTelephony and make sure the
         Telephony FWK can receive RIL_UNSOL_KEEPALIVE_STATUS.

Change-Id: I98f8d4a95a6c9c3c078a02aa9fd8722779623f12